Greetings, ac91dd69 looks like a forum troll, so dont mind.
Yes, you can only sell to the GM and don't buy with SC$ anymore, at least not directly.
You can buy stuff from your Worlds Market or build it in your country and sell on Space Stations for GCs.
The official reason for stopping GC Player-Player trade was that there were huge GC buy orders (some 1000 GCs each) from big players which couldn't be full filled so smaller orders from smaller players wouldn't be possibly successful. But that was when the GC price was capped to can only go downwards. So there were more and more orders but less and less supply as noone wanted to sell at those low prices.
The price of the GCs-buy orders from Andy lower frequently as general Prices of Products and so the total turnover is reduced.
Nowadays if you want to play more you have to pay more, even though the official statement at first was something like 'you won't need to pay more as we are working on a compensational gc income-source' namely Missions, well, never heard of them again ;)